Frequency modulation



H. TUNICK 2,296,962 FREQUENCY MODULATION Filed Dec. 22, 1959 ssheets-Sheet 1 /S i Q* TS Saw@ |El w` Jf www wm l INVENOR. H RRY TUN/CKATTORNEY.

Sept. 29, 1942.

\NNN WNNJ up" QWN @MN Filed Dec. 22, 1959 6 Sheets-Sheet 2 H. TUNlcK'2,296,962

FREQUENCY MODULATION Filed Dec. 22, 1939 6' Sheets-Sheet 5 Sept. 29,1942.

Sept. 29, 1942. H. TUNICK 2,296,952

FREQUENCY MODULAT ION Filed Deo. 22, 1939 6 Sheets-Sheet 4 750 7587 760,@762l I AM- DET AMPL 7 pfcr. l 754 z AMpL. Reer. DET' l 752 El TUBESB/AsED To c1/7'0F/=\ MAY BE 2y-PASS PARALLEL L//vso c/RcL//r Z/NED PPOS/TELY 70 OPPOSITE EX TREMES 0F BAND By-PAss 850 coNoL-'N- u 9.5/25

INV EN TOR.

i RRY TUN/CK BY v/WW A TTORN E Y.

Filed Deo. 22, 1939 6 Sheets-Sheet 6 INVENTOR. HARRY TUN/CK wwwATTORNEY.

Patented Sept. 29, 1942 FREQUENCY MoDULA'rIoN Harry Tunick, Rye, N. Y.,assignor to Radio Corporation of America,

Waffe a corporation of Deia? Application December 22, 1939, serial No.319,495

' 2o claims. (ci 25o-s) The inventions described herein relate tofrequency modulation systems and apparatus.

A f ew of the objects of my inventions are toprovide improved automaticfrequency controlling circuits for frequency modulation transmitters; toprovide' 'over-modulation indicating and controlling means for frequencymodulation transmitters; and to provide improved apparatus for eiectingfrequency modulation.

In the drawings:

Figure 1 is a wiring diagramof a frequency modulation transmitter havinga circuit for automatically bringing the transmitter back to its meanassigned frequency. Means are provided whereby this automatic correctionis operative only upon cessation of the modulating signal. Otherfeatures also are present.

Figure 1A is a wiring diagram of an over-modulation indicating andcorrecting system which may be applied to the transmitter of Figure 1Figure 1B is explanatory of Figure 1A.

. Figure 2 is another form of frequency mod-ulation transmitter.

Figures 3, 4 and 5 are wiring diagrams of other frequency modulationradio transmitters.

Figures 6 and 7 are characteristic curves ofampliers'which may be usedin conjunction with transmitters described herein and receivers forreceiving the transmissions from the transmitters.

Figure 8 is a block diagram of a frequency and phase modulationsignaling system.

In Figure 1, the signal pick-up device 2 feeds signal amplifier 4. Thepick-up device 2 may be a voice microphone or a television or facsimilepick-'up unit. If light signals are picked up, amplier 4 would be avideo amplifier. vThe amplified signal from amplifier 4 is fed throughtransformer 6 and lead 8 to the screen grids I0, I2 of reactance tubesI4, I6. In case video signals are transmitted, radio frequency oy-passcondenser I8 should be made smaller so as not to by-pass the highestvideo frequencies from amplifier 4 or, if desired, I8 may be omitted.

The reactance tubes I4, IB act to change the tuning of the grid circuit28, comprising coil 22 and condenser 24, an amount proportional to theamplitude ofthe signal fed through transmodulated high frequencyoscillations in the tank or plate circuit 34 comprising inductance coil3B and condenser 38. oscillations are fed through condensers 48, 42 to apower amplifier and/or limiter 44 and then to additional poweramplifiers and/or limiters and/or frequency multipliers 48 and radiatedsired, it may be coupled to the grid coil 22.` The' former 6 and at arate or frequency identical to I the frequency of the signal fed throughtransformer 6. As a consequence, the push-pull connected oscillatortubes 26, 28 regeneratively cross coupled by condensers 30, 32, producefrequency by antenna 48.

To prevent drift in the mean frequency of the push-pull oscillator26,'28 a coil 5I) is coupled to either the plate tank coil 36 as shownor, if deoscillator` energy picked up at coil 58 is 'fed to thesuppressor grids 52, 54 of tubes 58, 58 through leads 8 8, couplingcoil'64 and tuned circuit 62. Between the grids 64, 68 of tubes 56, 58there is connected a piezo-electric crystal 88 oscillating at somesuitable frequency higher or lower than the mean frequency ofoscillation of push-pull oscillation generator 26, 28. Tuned circuit 18connected. between the cathodes ofA tubes 56, 58 is tuned to thecrystalfrequency or to some harmonic thereof. The .screen grids 12, 'I4are grounded for radio frequency currents by the action of by-passingcondenser i8 and serve as the anodes for the oscillating system'formedby the crystal 88, tuned circuit 'I0 and the cathodes 'of tubes, 56, 58.Electrodes 18, 'i8 serve to screen the crystal controlled oscillatingsystem from the suppressor grids and plates of tubes 58, 58. All

of the electrodes of tubes 58, 58 are suitably polarized or biased asshown. The beat frequency oscillations resulting from inter-action ofcrystal controlled oscillations and those from the transmitter generatorfed through coupling coil 64 are resonated in the tunable plate circuit80. This beat frequency is preferably of a. lowerfrequency than eitherthe crystal frequency or the mean frequency of oscillator-tubes 26, 428,although if desired it may be higher than either the crystal frequencyor the frequency of oscilcoils s4, es are symmetncauy coup1ed to eitherc0il82 or to the coil of circuit 80 or to *The frequency modulated both.Coils 94,96 are arranged so that the electromotive forces inducedtherein are additiveand 'these electromotive forces are fed in push-pushto the midpoint 90 of resonant line 84. If deare symmetrically variablytapped, as shown, to

the resonant line 84 and excite the grids |,I4, ||6 of the detectorsII8, |20. The grids of'detectors II8, |20 are biased through lead |22and tubes could be taken in a similar fashion from the plate circuit' 34as was done from the grid circuit 20 and, of course, the cathodes orfilapotentiometer |24, suitable chokes |26, |28 being provided toprevent high frequency currents from passing into lead |22. A resistance|30 is connected between the plates of detector tubes |I8, and shuntedacross each portion of re,

sistor are time constant 'condensers |32, |34.

By virtue of the foregoingarrangement, should the frequency ofoscillation generator tubes 26, 28 drift from'its mean assigned valuefor any reason, the rectified current output of tubes II8 or |20 will belarger depending upon the relative drift .in frequency of oscillationgenerator tubes 26, 28. 'This' change in frequency produces a voltagedrop across leads |36, |38 which is added to or subtracted fromtheVoltage from potentiometer |40 fed through resistance I 42 to thesuppressor grids |44, |46 of reactance tubes I4, I6. Reactance tubes I4and I6 are drawing lagging or leading currents depending upon theadjustments of condensers |46, |50 and resistors |52, |54. Inj-'otherwords, by virtue of condenser |48 and resistance |52 the voltageAimpressed upon grid |56 of tube I4 is such as to make that tube draweither a leading or lagging current with re.

.ments of these additional tubes would be grounded. This m'odicatiominwhich another pair of reactance tubes is connected vto the plate circuit34, is illustrated in Figure 1C.

As shown in Figure 1C, the suppressor grids RI44 and RI46 of the platecircuit reactance tubes RI4, RIG are. connected in parallel and to thesuppressor grids |44, |46 of reactance tubes tion produced by thesignal, part of the signal spect to the current in the grid circuit 20.So

such a voltage as to cause tube I6 to draw either a lagging or leadingcurrent.4 The voltage impressed upon the suppressor grids |44, |46through leads |36, I 38 andresistor |42 is made to be in suchadirection'as to correct for de- A partures in frequency of oscillation generator26,

28 from its mean assigned frequency and to bring it back to thatvmeanassigned frequency for which the voltage difference across leads |36,|38 as derived from resistor |30 is zero or negligible. The plates ofthe reactan'ce tubes I 4, I6 may be supplied with potential from anysuitable source such' as |60 feeding the plates through choke coils'|62, |64. Bypassing condensers |66, |68 may be provided if desired.Also, blocking condensers |10, I'I2 are used to connect the plates ofthe reactance tubes to the grid circuit 20.

Frequency modulation of the oscillations produced by tubes 26, 28 isaccomplished by impressing signal voltages through transformer 6 Vandlead 8 upon grids I0, I2, thereby effectively varying the amount ofleading or lagging current 'drawn by the reactance tubes in accordancewith system 26, 28.

If desired, another pair of reactapce tubes may be connected similarlyto the plate circuit 34 of the push-pull oscillation generator, in whichcase the suppressor and screen grids would be connected in parallel tothe suppressor'and screen grids of tubes'I4 and I6. Phase displacedvoltalso for reactance tube I6, the condenser |50 and resistor l|54cause grid |58 to be subjected to' voltage may be fed through switch|80, signal amplifier |82 and rectifier |84, Whose rectified voltageisimpressed across the resistance condenser .time constant circuit |86.Arrangements should be made as regardsthe amplifying power of theamplifier I 82 and voltage developed by rectifier |84 so that in thepresence ofthe lowest value of signal desired to be transmittedithevoltage impressed by rectifier |84 in series with the voltage frompotentiometer |24 is such as to bias detector tubes |20, |I8 to cut off.In this way, when the device 2 picks up signals to be transmitted, thefrequency controlling circuits are rendered inoperative. Obviously, thesystem of Figure l may be operated without this auxiliary control byopening switch |80, in which eventautomatic frequency control of thesystem will take place at all times. With switch |80 closed, frequencycorrection takes place only in amplitude changes in the oscillationgenerator is ages on the grids ofthese additional reactance fed fromtime-constant circuit |94 and leads |96 to, resistor- .|98 in serieswith the grid biasing lead 200 connected through coil 22 to the grids202, 204 of the push pull connected tubes 26, 28. Leads |96 should be soconnected that-the voltage inserted in series\ with thegrid biasing lead200 from the fast a. v. c circuit |94 is in such a direction as to wipeout amplitude changes in the oscillation generating circuits to whichcoil |92 may be coupled. If desired, of course, coil |92 may be coupledto an output circuit of either limiter 44 orl an output circuit or inputcircuit of circuits =46 should circuits 4 4 or 46 imperfectly eliminateundesired amplitude variations.

Leads y2|0, 2I2 -of Figure 1 are identical to leads 2|0, 2I2 of Figure1A which illustrates an over-modulation system for a frequencymodulation transmitter. Assuming, as shown in Figure A 1B, that thefrequency channel allocated to a frequency modulation transmitter isrepresented by-the frequency range between points A and:B, then tunedcircuits 2|4, 2|6 connected to. the coupling antennas 2|8, 220 are tunedso as to have resonant points R, R respectively at or slightly beyondthe frequencies A-and B. AAs a consequence, should the transmitterfrequency be shifted an amount greater than the deviations A and B awayfrom the mean assigned frequency M ofFigure 1B, current will be inducedin the amplifiers 222, 224l in turn coupled to rectifiers 226, 228. Therectifiers are connected so as to feed current through resistor 230 inseries with the polarizing battery or source 232. In the presence ofover-modulation, leads 210, 212 are so connected as to inject a voltagein resistor 234 (Fig.

l) in' series with-the signal lead 8 in such a di` rection as to cutdown the amount of frequency swing of the oscillation generating system.This auxiliary control may be eliminated, -if desired, by opening switch236. In the alternative, or in addition to this auxiliary control, abell 238 and connected .through lead 244 to resistance 230 of Figure 1A.

In the transmitter of Figure 2, microphone 300 feeds an audio frequencyamplier 302 whose characteristic is as shown in Figure 2A. That is,amplifier 302 should have a fiat 'characteristic along the range Ato Bof about to 500 or 10 to 1000 cycles. From points B to C thecharacteristic should rise preferably linearly. 13 to C represents theAremainder of the audio frequency range which may be as desired from 500or 1000 to-5000, '1500 or- 10,000 or more cycles. When such an amplifieris used at the transmitter, the frequency modulation audio frequencyamplifier at the receiver should have a complecomes active to produce anindication in the bell or lamp indicator 354 and also inject a' gainreducing voltage in the automatic volume controlling lead 356 leading tothe same or another a light 240 may be actuated through relay 242amplifier stage in audio frequency amplifier 302. If desired, the systemmay be operated without the automatic volume controlling feature, inwhich case switch 360 and/or switch 362 may be opened.

Incidentally, in connection with Figure 1, the

condenser of tuned circuit 80 may be omitted.

and, if desired, may be replaced by a resistance to broaden the passband of energy fed into coil 82.

In Figure 3 I ,have illustrated another irnproved frequency modulationtransmitter. The crystal 100 controls the frequency of'oscillation ofthe crystal controlled oscillator tube V102 which is-provided with aninductive output circuit or mentary characteristic, namely, fiat alongthe frequency vrange from A to B and then :drop o from the frequencyrange B to C, as shown in Figure 2B.

The output of audio frequency amplifier 302 is fed through transformer304 and additional transformers 306, .308 to thegrids of reactance tubes310, 3 I2. Audio frequency by-passing con-y densers 314, 3I6 areprovided as are also radio frequency chokes 3I8, 320. Depending upon theadjustments ofjreactance tubes 310, 3I2 the crystal 322 is effectivelyshunted by variable capacity or inductance represented by tube 310andthe plate circuit ofthe crystal controlled oscillation generator tube324 is also shunted by a variable inductor or capacity represented byreactance tube 3I2. Both Vtubes 310, 3I2 should be adjusted so as toaddA similar reactances,

namely, capacitive or inductive, to the grid and plate circuits ofcrystal controlled generator 324.

If desired, of course, thecrystal may be replaced by a tuned circuit ora quarter wave resonani; line. The audio frequency voltages applied-tothe reactance tubes 3|0, 3|2 may be adjusted by virtue .of taps 330, 332on the potentiometer shunting the secondary of transformer 304. Theoutput of the oscillation generator may be fed through a limiter and/orpower amplifier and/or frequency -multiplier 334 whose output asradiated by the antenna.336.

As before, over moduation pickup antennae or coils 338, 340 areprovided, antenna 338 feeding into a circuit 342 tuned to or slightlybeyond one 4extreme frequency of the channel assigned to the transmitterof Figure 2. Similarly, tuned circuit 342 is tuned to or slightly beyondthe other extreme frequency of the channel assigned to the transmitter.Circuit 340 energizes amplifier 344, in tum connectedto rectifier 346.

' When the swung exceeds the channel allotted to the transmitter, anindication is produced in 'indicator l340 and to automatically preventthis coil 104 which may be shunted; if desired, by a tuning' condenser.In the absence of signal from the signal amplifier 106,.tubes 108, are,biased to cut off by voltage source 112. Tubes 108 and 1I0 are madealternately conducting with audio frequencies transferred throughtransformer 114. Coil 1|6 is so wound as to increase the inductancereected into coil 104 with v'current flow through tube H0 and coil 118is so wound and polarized as to decreaseby refiection the effectiveamount of inductance in coil 104. Consequently, the frequency ofoscillation of oscillation generator tube 102 is varied in accordancewith signals from amplifier 106. The output of the oscillator is fed toApower amplifiers and/or limiters and/or frequency multipliers and thenradiated over antenna 132. The crystal may, of course, be replaced by aparallel tuned circuit by throwing switch 142 to its upper position 144.

Preferably, by adjusting the volume of signal fed to the varioustransformers described herein and by suitable adjustment and choice ofthe frequency multipliers, I prefer to radiate a wave whose frequency isequal to, for the loudest signals, any integer up `to one hundred timesthe highest modulating frequency plus the highest modulating frequency.Thus, if we assume the highest .modulating vfrequency to be 5000 cyclesand the integer Ato be, for example, five, then the.radiated carrierfrequency deviation should be five times 5000 plus 5000 or 30,000cycles, making the total band width used by the transmitter 60,000cycles. For the case where the integer is, for example, 100, thedeviation is 500,000 plus 5,000 under the above rule,

thereby making the total band width employed .when frequency modulatingwith speech waves,

the transmitter may go beyond one .extreme of its assigned channel andnot beyond the other,

As one'iway of remedying this situation, I have provided in connectionwith the transmitter of Figure 3 two pick-up antennae 150, 152 coupledto the radiatingantenna 132. Antenna 150 feeds tuned circuit 154, tunedto or below the lowest frequency of the frequency channel assigned tothe transmitter 102, 130, 132 and tuned circuit 158 is tuned to or abovethe highest frequency of the channel assigned'to the transmitter ofFigure 3. Tuned circuit 154 feeds amplifier rectifier 158 and detector160. The detected output, when the frequency of the transmitter goesbeyond the lower limit of the,assigned channel, causes indication in anindicator 162 and also impresses a voltage across resistor 164 in serieswith the grid 168 of a tube 1| 0. The voltage impressed in resistor 184is in such a direction as to cut down current flow through tube 1|0 and,hence, reduce the effective inductance reflected into coil 104 by coil1I6. 'I'hat is, assuming coil 1I6 withl increasing current therethroughcauses an effective increase in inductance of coil 104, Vover-modulation in one direction as detected by the action of coil 154 willmanifest itself through the voltage drop in resistance 164 to reduce thefrequency swing in that direction in which overmodulation is beingproduced.

Coil 1I8 acts effectively to reduce the effective inductance of coil 104and hence increases the frequency of oscillation of tube 102 withincreasing current flow through coil 1|8. When the radiated wave fromantenna 132 goes above its I upper assigned frequency current will beproduced 840 is identical toqthat within the dotted rectangle 842. Bythe action-of switch 844 either the crystal 848, parallel tuned circuit848 or reso-,-y nant line 850 is connected inl the grid circuitfof thehigh/ frequency oscillation generatdrm852. The output of the generator852 vis fed 'through power amplifier and/or limiter and/or frequencymultiplier to antenna 856.

` Obviously, both reactance tubes 800, 802 may be adjusted so that theyboth draw leading currents or both' draw lagging currents. Preferably,

coil 8|6 is omitted and condensers 8|,2, 8|8 are made identical in sizeso that both reactance tubes 800, 802 act alike.

-In the transmitter of Figure the oscillator tube 800 may have eithercrystal 802 connected lbetween the plate 804 and grid 806 or, as shown,

` the series tuned circuit 808 is Aconnected by means of switch 8|0between the plate 804 and v grid 808. The screen grid 820 is suppliedwith voltage through radio frequency choke 822 and by-pass condenser824. The plate 8044 is supin tuned circuit 152 and the current in thiscircuit will be amplified by amplifier 180 and rectified by rectifier182.- This rectified current produces an indication in the indicator 184and also produces a voltage drop across resistor 185 in series with thegrid 188 of tube 108. This voltage drop should be arranged in such a wayas to reduce the current flow through tube 108 and hence through theaction of coil 1|8 upon coil 104 bring the frequency of oscillation oftube 102 WithinA its assigned channel.

plied with plus voltage through choke 830 by passed by by-passingcondenser 832. lTransformers 834 and 838 insert alternating signalvoltages in the plate and screen voltage leads. 'I'hese signal voltagesare derived from potentiometer 840 connected to the secondary oftransformer 842 whose primary is connected across the plates ofpush-pull amplifier 844 energized from audio or signaling amplifier 946.Since like voltage changes applied to the screen 820 and plate 804produce opposite effects on the frequency of oscillation of tube 800transformers 836 and 834 are so polarized as to oppositely swing thescreen 820 and plate. 804 inl voltage in Incidentally, it is to be notedthat` the unsymmetrical characteristic of the amplitude timecharacteristic of speech waves may cause the automatic frequencycontrolling system of Figure 1 to drag the oscillator over a period-.oftime to one side of its mean assigned frequency especial-- ly when thesystem of Figure -1 is operated with switch |80 open. To correct forthis, taps 880, 882 of Figure 1 may be unsymmetrically tapped about themidpoint of resistor |30 until the unsymmetrical characteristic'of themodulation in.. troduced at point 2 is compensated. To facilitate vthisadjustment, bearing -in mind the unsymmetrical characteristic of speechwaves, a reversing switch 884 (Figure 1) may be provided for reversingthe polarity of the signal fed into lead 8 through transformer 8.

In the transmitter of Figure 4, reactance tubes 800, 802 are made moreconductive by the action of transformer804 associated -with-the cathodesof reactance tubes 800, 802. Transformer 804 is energized by signalamp1iflerv808 in turn connected, if desired, to a'microphone 8|0.lCondenser '812 and resistor 8|4 are adjusted so as to energize the grid8I6 with a relatively leadingA voltage. Condenser 8I8, coil {H6-andresistance 820 are adjusted so as to impress a relatively laggingvoltage with respect to the voltage at point 822 uponthe grid 824 oftube 802.l The cathodes of the reactance tubes are connected to a sourceof. potential 830 such that both tubes draw a mean current in theabsence of signal from amplifier 808. The apparatus Within boxaccordance with signal voltages derived from amplifier 846. The outputcircuit 850 of the oscillator 800 is fed to the limiter 852 which may bea power amplifier and/or frequency multiplier and in turn 852 energizesthe antenna 854.

If desired, the audio frequency amplifier at the transmitter, such asthe amplifier 4 of Figure l or the amplifier 302 of Figure 2 or theamplifier- 106 of Figure 3, may have a Acharacteristic such as shown inFigure 6, in which case the receiving audio frequency amplifier shouldhave an amplitude frequency characteristic such as shown in Figure 7. Inthe transmitter amplifier characteristic of Figure 6, the gain is flatfrom about 50 to 100 cycles, rises linearly from about 100 to 500cyclesand then rises according .to the sloping characteristic shown from500 cycles on. The receiving characteristic of Figure 'Lis-complemen-`tary to that of Figure 6 and is fiat from 50 to ,100 cycles, decreasesthe gain linearly from to about 500 cycles and then slopes off in gainas shown from 500 cyclesup. s u

Clearly, many of the features of my present invention are applicable tophase modulation systems. For example,v the over-modulation indicatingsystem may be used exactly as shown for a A phase modumnon transmitter.A130, au' of the frequency modulation transmitters described herein maybe caused to produce phase modulated waves by adding an additionalcorrecting circuit in the audio stages of the transmitter which servesto increase the audio amplifier out- 1|8 are preferably tuned bycondensers 1|6' sub-carrier system 3002. The output offrequency or phasemodulation receiver 3006 is then used to frequency or phase modulate theradio transmitter 3008 whose output is in turn radiated by radiator30I0.

The transmitted frequency modulated waves are received on thereceivingantenna 30l2 and translated by the frequency or phase modulated receiver30M into the modulation frequency waves corresponding to those at theoutput of apparatus 3000 at the transmitter. These de-modulated waves ofmodulation frequency are used to frequency or phase modulate asub-carrier wave in the apparatus represented by 3016. This frequency orfed over land line 30|8 to a distant receiving point at which afrequency or phase modulation receiver 3020 is located. Thereceiver 3020de.

modulates the frequency or phase modulated subcarrier and feeds thede-modulated waves into a loudspeaker facsimile recorder or televisionreproducing apparatus 3022. The foregoing system has among motelylocated from the transmitting antenna 3010 and that the de-modulatingreceiver 3006 f may be used bothfor' monitoring as well as formodulating the radio transmitter 3008. Similarly, the receivingarrangement is advantageous which event-the urban noises generated inurban areas are kept from affecting the output of 3022.

Any of the transmitters or receivers-heretofore specifically describedmay, of course,be used in the system of Figure 8, as will' be apparentto those skilled in the art.

In connection with Figure 3, as shown on the drawing, the coils 116, 1I8are preferably shielded and uncoupled from Veach other althoughindividually coupled to the coil 104. Also, coils 1|6,

1|8' to opposite extremes of the frequency band transmitted byoscillation generator 102. From another point of view it may be saidthat these off tuned circuits 1|6, 116 and 1|8 1|8 serve to swing theresonant frequency of the oscillation generator plate circuit 104through and about its mean frequency in accordance with the modulationfed through transformer 1H.

In connection with Figure. 5. the relative voltages applied to the plate904 and screen grid 920 may be adjusted as shown by the taps on resistor940 and/or by adjusting transformers 934. 936.

Instead of manually reversing the polarity of the signal modulationinput for the transmitting system shown, for example, in Figure 1 thepolarity thereof may be automatically reversed by systems such as shownby J. L. Hathaway in his U. S. Patents 2,158,820 and 2,158,821.

In the claims which follow, the term "angular velocity modulation shallbe understood to include frequency modulation or phase modulation orhybrid modulations suchv as produced by predistorting or pre-emphasizingcircuits at the transmitter. y

Having thus described my invention, what I claim is:

l. In combination, an oscillation generator, a source of signals, meansfor angular velocity phase modulated sub-carrier is then of a city andthe reproducing apand its advantages the fact that the pick-up unit 3000may be reassigned frequency, and means, responsive to signals to betransmitted. for disabling or rendering inoperative said automaticfrequency correcting means.

2. Apparatus as claimed in claim 1, characterized by the fact that meansare provided responsive to signaling waves for rendering said automaticfrequency controlling means inoperative in the presence of signals andoperative in the absence of signals.

3. An angular velocity modulation over-modulation system comprising acircuit tuned to or beyond an extreme frequency of the frequency channelto which an angular velocity modulation transmitter has been assigned,saidtuned circuit being coupled to said transmitter so that usablecurrent ows in said circuit only when'said transmitter produces waves offrequencies exceeding the channel assigned to said transmitter, meansfor rectifying the current produced in said tuned circuit, and means forindicating presence of said rectified current, said apparatus beingcharacterizedby the fact that means are provided responsive to saidrectified current for reducing the frequency swing of the transmitter sothat it does not produce angular velocity modulated oscillations of afrequency extending beyond the frequency to which said circuit is tuned.

4. An angular velocity modulation over-modulation system comprising acircuit tuned to or beyond an extreme frequency of thefrequency channelto which an angular velocity modulation transmitter has been assigned,said tuned circuit being coupled to said transmitter so that usefulcurrent flows in said circuit only when said transmitter produces Wavesof frequencies exceeding the channel assigned to said transmitter, meansfor rectifying the current produced in said tuned circuit, saidapparatus being characterized by the fact that means are providedresponsive to said rectifiedA current for reducing the lfrequency swingof the transmitter so that it does not produce angular .velocitymodulated oscillations of a frequency extending beyond the frequency towhichsaid circuit is tuned.

5. In an angular velocity modulation system, an angular velocitymodulated transmitter, a pair of tuned circuits tuned to or beyondextreme frequencies of an assigned frequency modulation.

channel, means for rectifying currents produced in said tuned circuits,and means for utilizing the rectified currents to reduce the frequencyswing of said transmitter so as to confine the angular velocitymodulated waves produced thereby within said assigned or allocatedfrequency channel. 6. In a frequency modulation system, an oscillationgenerator, means for frequency moduized by the fact that saidoscillation generator is provided with automatic frequency controllingmeans, and being further characterized by the fact that said automaticfrequency controlling means is adjusted in such a way as to compensatefor the unsymmetrical nature of voice waves used to produce thefrequency modulatedoscillations in such' manner that the frequencycontrolling system brings the oscillation generator back to its meanassigned frequency rather than 'on the average to a different` frequencywhich source of oscillations to be modulated, a reactance tube connectedto said source, said reactance tube having a multiplicity of electrodes,means subjecting one pair of said electrodes'to phase displacedoscillations from said source, mean'l subjecting another pair ofelectrodes of said reactance tube to signaling current waves, and meansfor subjecting still another pair of electrodes of said reactance tubeto automatic frequency controlling voltages derived from said source ofoscillations, and characterized further by the fact that a pair ofelectrodes of saidreactance t'ube is provided with and subjected to overmodulation control voltages. Y

8. In combination, asource of complex signal modulating waves, anamplifier to amplify said waves, an oscillation generator, a reactancetube circuit coupling said amplifier and generator whereby the Wavesderived from said generator are angular velocity modulated in accordancewith waves from said source, a pair'of frequency discriminator circuits,a pair of rectifiers connected to said discriminator circuits, saiddiscriminator circuits being energized by modulated Waves derived fromthe generator and being adjusted so that said rectifiers producesubstantial rectified voltages when said generator is overmodulated, andmeans connecting said rectiflers to saidreactance tube circuit in such away as to produce a modulation to oppose the signal modulation only whenover-modulation occurs.

9. Apparatus as claimed in the preceding claim, characterized by thefact that the rectifiers are connected to the signal amplifier to reducethe gain thereof when over-modulation occurs.

10. Apparatus as claimed in claim 8, characterized by the fact that therectifiers are connected to the reactance tube circuits to. reduce theamount of angular velocity modulation when over-modulation occurs.

11|. In combination, a generator of carrier waves. a source of controlwaves, means to anell- 'lar velocity modulate carrier waves from saidgenerator with waves from said source, and means responsive to themodulated waves for reducing the degree of angular velocity modulationthereof only when the degree of modulation exceeds pre-determinedlimits.

12, In combination, a generator of carrier Waves,a source of modulatingwaves, means. to angularyelocity modulate waves derived from saidcarrier generator with waves from said modulating Wave source, and meansresponsive only to over-modulated waves for reducing the amount ofover-modulation.

.13. In combination, an oscillation generaton-a over-modulation of saidgenerator over-modulation thereof.

14. In combination, a generator of carrier waves, circuits fortransmitting waves derived from the generator, a source of variablecontrol voltage, circuits arranged to frequency modulate waves derivedfrom said generator in accordance to reduce the .with said variablecontrol voltages, a frequency controlling circuit arrangement forfrequency stabilizing lsaid generator: a rectifying system, operative toproduce rectified current only when the frequency modulated waves swingbeyond desired frequency limits, to produce a voltage representative ofthe excessive frequency swing, and

said generator operating at a substantially constant mean frequency, anda second discriminator-detector system responsive to over-modulation ofsaid generator for reducing said overmodulation. i

16. In a system for controlling the maximum l deviation of frequencymodulated oscillations irrespective of changes in the average intensityof said signals, means for producing rectied current only when thedeviations of the modulated oscillations exceed a selected frequency,the intensity of said` rectified current being proportional to theextent by which the frequency of Vsaid modulated oscillations exceedssaid selected frequency, and means excited by the rectified currentproduced for changing the mean frequency of said oscillations as afunction of .said

rectified current, said last-mentioned means be ing operative only whensaid maximum deviation -is exceeded.

17. The method of controlling the peak deviations` of oscillationsmodulated in frequency by voltages the average value `of which may varyslowly which includes the steps of producing, only from undesired,excessive frequency deviations, additional voltage the value of which isa function of the extent which the peak deviation of said modulatedoscillations .exceed a selected value during modulation, and changingthe mean frequency of said oscillations in accordance with saidadditional voltage.

18. In a frequency modulator including an oscillation generator, areactance tube modulator coupled thereto and means for controlling theimpedance of said reactance tube by signals,

means for deriving from said modulated oscillations a potential solelyfrom peak swings thereof which go beyond a selected frequency, and meansfor additionally controlling the impedance of said reactance tube inaccordance with said derived potential.

19. In combination, a generator of carrier waves, a source of controlvoltages, means to v frequency modulate carrier Waves from saidgeneratorv in accordance with voltages from said source, a frequency toamplitude converting circuit, a rectifier coupled thereto, means forfeeding frequency modulated waves to said converting circuit andthenceto said rectifier, anda circuit coupling the output of said rectifier tosaid generator, said converting circuit and rectiiier operating toreduce the degree of frequencyi modulation of the carrier waves onlywhen the degree of modulation exceeds predetermined limits.

20. In combination, a generator of carrier waves, a source of modulatingwaves, means to frequency modulate waves derived from said generatorwith waves from said modulating wave source, a'frequency to amplitudeconverting and detecting system responsive only to over-modulated wavesfor producing detected current only when over-modulation occur's, and acircuit utilizing said detected currents for reducing the de- 5 gree offrequency modulation of said carrier waves when over-modulation occurs.

HARRY TUNICK.

